Summary Impaired fracture healing place considerable financial, physical, and mental burdens on individuals, especially in the geriatric population, which renders the need for new therapies to accelerate healing of paramount importance. With the growing evidence that the gut microbiota regulates skeletal homeostasis, we asked whether the beneficial effects of probiotic intake on bone health may be similar or different in the case of traumatic bone injuries. Thus, while it is widely accepted that bone mineral density only predicts fracture risk, and is not indicative of bone healing, the effects of microbiota on bone modeling and systemic inflammation in the context of fracture healing remains unexplored. Moreover, whether a single probiotic can positively impact senile fractures has also not been previously reported. Therefore, the overall goal of this proposal is to establish the gut microbiota as a modulator of the inflammatory as well as the osteogenic responses to fracture healing in the context of aging. In our preliminary data section, we show that that dietary supplementation with the candidate probiotic Bifidobacterium adolescentis alters the profile of serum cytokines, protects against fracture induced systemic bone loss, increases fracture callus bone volume, and accelerates the healing sequelae. We therefore hypothesize that the microbiome is a key regulator of secondary bone healing through the modulation of systemic innate inflammatory signals, and that dietary supplementation with probiotics that modulate these inflammatory signals may be used as therapeutic modalities to accelerate fracture healing. While growing evidence points to beneficial effects of probiotics on skeletal homeostasis, their role in skeletal repair and regeneration remains elusive. Here we propose to first compare fracture healing rates between germ- free and conventional mice, and to determine whether this will be influenced by colonization with aged- microbiome (Aim 1). The current dogma posits that the gut microbiota regulates bone mass through mechanisms involving shifts in inflammatory cell populations and their respective cytokines as well as through maintenance of the intestinal barrier. In the second Aim or this proposal, we will establish the relevance of the gut microbiota to induce changes in the gut leakiness responsible for exacerbated inflammation observed during aging and in the context of traumatic bone healing. Thus, we will establish the therapeutic efficacy of probiotic manipulation of the gut microbiome for accelerated secondary bone fracture repair in aged mice (Aim 2). Discovering specific bacterial taxa within the intestine that accelerate bone healing will be essential information for the characterization of a eubiotic microbiome that may elicit optimal fracture repair rates. Moreover, this information will enable the development of therapeutic approaches whereby the identification of gut microbial obstructions to fracture healing within the gut microbiome of geriatric population may be treated by supplementing the diet with probiotics, or with antibiotic therapy. Altogether, these studies will identify novel treatment modalities to enhance the rates of fracture healing in patients.!